The present invention relates to image forming apparatuses such as printers, copying machines, facsimiles or complex machines that have the functions of them in a complex form and to fixing devices and conveying devices for use in the image forming apparatuses.
In an electrophotographic image forming apparatus, a developer carried on a sheet that is an image carrier is fixed to the sheet by applying heat and a pressure to it. For the above purpose, a roller made of a solid rubber or the like has conventionally been used as heating means for heating the sheet and the developer in a fixing device incorporated in the image forming apparatus. However, a belt-type fixing device employing a fixing belt that has an excellent thermal efficiency and a smaller thermal capacity than that of the roller to allow the temperature to rise in a short time and is consequently able to shorten a warmup time has lately been used.
One example of the belt-type fixing device is disclosed in JP 2006-189690 A. In this case, the belt-type fixing device disclosed in JP 2006-189690 A has an endless fixing belt, a fixing roller and a heating roller that are located inside the fixing belt and support the fixing belt, and a pressure roller that is located outside the fixing roller and holds the fixing belt with the fixing roller. The fixing roller and the pressure roller are designed to have the same length in the lengthwise direction. A heat source (halogen lamp) is built in the heating roller and the pressure roller. Therefore, the heat sources of the heating roller and the pressure roller are supplied with an electric power during the fixing, so that the sheet and the developer supplied in between the fixing belt and the pressure roller are heated by heat supplied from the heating roller and the pressure roller heated by the heat sources, fixing the developer to the sheet.
The outer peripheral surfaces of the rollers such as the fixing roller and the pressure roller used for the belt-type fixing device as described above are each covered with an elastic layer. The elastic layer, which has excellent deformability, forms a large contact region (nip portion, nip region) between the fixing belt and the pressure roller. With this arrangement, a sufficient fixing time can be secured when the sheet is conveyed at high speed. Moreover, the elastic layer of the fixing roller is softly set, and the elastic layer of the pressure roller is set harder than that of the fixing roller, so that the nip portion is formed curved convex to the fixing roller. With this arrangement, the leading end of the sheet discharged from the nip portion is directed toward a direction in which it is separated away from the fixing belt, so that the sheet can easily be separated from the fixing belt.
However, when a belt of a laminate structure including a metal layer is used in the belt-type fixing device that has the hard pressure roller and the soft fixing roller as described above, it is concerned that an excessive stress is given to the belt depending on the contact condition of the hard pressure roller and the soft fixing roller. For example, the soft elastic layer in the outer peripheral portion of the fixing roller is expanded in the lengthwise direction thereof, and the edge portions of the belt are bent toward the pressure roller. As a result, fixation of the developer is insufficient in both side end portions of the sheet, or a disorder of the image occurs as a consequence of the outward movement of the unfixed developer toward the end portions. Since developers of a plurality of colors are layered on the sheet particularly in a full-color image forming apparatus, the disorder of the image often occurs. Moreover, when the fixing belt includes a metal layer (electromagnetic induction heating layer), an excessive bending stress is generated in the metal layer, possibly causing cracks.